A lithium-ion secondary battery which is representative of a non-aqueous electrolyte secondary battery has properties such as lightweight, high electromotive force and high-energy density. Accordingly, the lithium-ion secondary battery has been increasingly used as driving power sources of various types of portable electronic apparatuses or mobile communication apparatuses such as a mobile telephone, a digital camera, a video camera and a notebook type computer.
The lithium-ion secondary battery includes a positive electrode made of complex oxide containing lithium, a negative electrode including a lithium metal, a lithium alloy or a negative electrode active material inserting/extracting lithium ion, and an electrolyte.
Recently, instead of a carbon material, such as graphite, which has been conventionally used as a negative electrode material, research into an element having insertion property of a lithium ion and having a theoretical capacity density of more than 833 mAh/cm3 has been reported. For example, as an element of a negative electrode active material having the theoretical capacity density of more than 833 mAh/cm3, there is silicon (Si), tin (Sn) or germanium (Ge) which is alloyed with lithium, oxide or ally thereof. Among them, since silicon-containing particles such as Si particles or silicon oxide particles are cheap, they have been widely investigated.
However, the volumes of these elements are increased when lithium ions are inserted during charging. For example, if the negative electrode active material is Si, Li4.4Si is obtained in a state in which a maximum amount of lithium ions is inserted. Since Si is changed to Li4.4Si, the volume thereof is increased to 4.12 times of a discharging cycle.
Accordingly, if a thin film of the element is deposited on a current collector by a CVD method or a sputtering method so as to form a negative electrode active material, the negative electrode active material expands or contracts by inserting/extracting the lithium ion and peeling may occur due to deterioration of adhesion of the negative electrode active material and a negative electrode current collector in repeated charging/discharging cycle.
In order to solve the above-described problems, a method (for example, see Patent Document 1) of forming irregularities in the surface of a current collector, depositing a negative electrode active material thin film thereon, and forming a space in a thickness direction by etching was disclosed. In addition, a method of providing a mesh on a current collector, depositing a negative electrode active material thin film through the mesh, and suppressing the deposition of the negative electrode active material in a region corresponding to a frame of the mesh was suggested (for example, see Patent Document 2).
In addition, a method of forming irregularities on the surface of a current collector and obliquely forming a thin-film-shaped negative electrode material with respect to a surface perpendicular to a main surface of the negative electrode material was suggested (for example, see Patent Document 3).
In the secondary battery disclosed in Patent Document 1 or Patent Document 2, the negative electrode active material thin film is formed in a columnar shape and the space is formed between the columnar, thereby preventing peeling or wrinkles. However, since the negative electrode active material contracts in the start of charging, a metal surface of the current collector may be exposed through the space. Accordingly, since the exposed current collector faces a positive electrode during charging, lithium metal is susceptible to be precipitated and thus stability or capacity may deteriorate. In order to increase the capacity of the battery, if the height of the negative electrode active material having the columnar shape is increased or the interval between the spaces is decreased, a front end (an opening side) of the negative electrode active material having the columnar shape is restricted by the current collector and thus, as the charging progresses, the negative electrode active material significantly expands compared with the vicinity of the current collector. As a result, the negative electrode active materials having the columnar shape are brought into contact with each other in the vicinity of the front end thereof and thus the current collector and the negative electrode active material are peeled or wrinkles are generated in the current collector due to pushing. Accordingly, it is impossible to simultaneously realize high capacity and the prevention of the peeling of the current collector and the negative electrode active material or the generation of the wrinkles in the current collector. In addition, since an electrolyte is filled in the space between the columnar-shaped negative electrode active materials which expand and contact with each other, the movement of the lithium ions at the beginning of discharge is blocked and, more particularly, a high-rate discharging or a discharging characteristic in a low-temperature environment is problematic.
In the structure disclosed in Patent Document 3, as shown in FIG. 13A, it is possible to prevent current collector 551 from being exposed and prevent the lithium metal from being precipitated, by negative electrode active material 553 formed obliquely θ. However, similar to Patent Documents 1 and 2, as shown in FIG. 13B, since negative electrode active material 553 significantly expands as the charging progresses compared with the vicinity of current collector 551, the negative electrode active materials having the columnar shape are brought into contact with each other in the vicinity of the front end thereof and thus current collector 551 and negative electrode active material 553 are peeled or wrinkles are generated in current collector 551 due to pushing, as denoted by an arrow of the drawing. In addition, since the negative electrode active material is obliquely formed, the negative electrode active material is formed on only two surfaces of a longitudinal direction of a protruding portion of the current collector. Accordingly, stress due to the expansion/contraction of the negative electrode active material in charging/discharging cycle should be lessened by the negative electrode active material covering the two surface of the protruding portion. As a result, as the charging/discharging cycle progresses, the negative electrode active material is susceptible to be peeled from the surface of the protruding portion by the stress and reliability deteriorates. In addition, since an electrolyte is filled in space 555 between the columnar-shaped negative electrode active materials which expand and contact with each other, the movement of the lithium ions at the beginning of discharge is blocked and, more particularly, a high-rate discharging or a discharging characteristic in a low-temperature environment is problematic.    [Patent Document 1] Japanese Patent Unexamined Publication No. 2003-17040    [Patent Document 2] Japanese Patent Unexamined Publication No. 2002-279974    [Patent Document 3] Japanese Patent Unexamined Publication No. 2005-196970